1. Field of the Invention
This invention describes ring laser cavities which incorporate optical amplifiers to generate and amplify light which is propagating in the ring. The invention more particularly relates to ring laser cavities where light is amplified by a large area optical amplifier and the spatial and spectral properties of the amplifier emission are controlled using optical elements in the cavity.
2. Background Information
Large active area lasers, such as coupled-stripe laser diode arrays having many narrow stripes are able to generate much greater radiation power than single narrow stripe laser diodes. Broad-stripe laser diodes can generate much more radiation power than can typical narrow stripe laser diodes. Both the arrays and the broad stripe diodes are called large area lasers because of their large active area, which accounts for their high radiation powers.
Undesirable characteristics of large area lasers limit their usefulness. The radiation from these lasers is wide lobed rather than narrow lobe and the spectrum of the emission is multilongitudinal mode rather than monolongitudinal mode. These characteristics prevent their use in applications such as coherent optical communications where narrow optical spectrum and diffraction limited beam focusing are required.
The wide-lobed pattern of the emission in either case causes the laser emission to be especially undesirable. The shape makes it impossible to efficiently couple the radiation from the large-area laser into a single mode optical fiber and makes it impossible to achieve a diffraction-limited collimated beam needed in free space propagation communication systems.
Single mode and single diffraction limited lobe operation of a large area laser (i.e. a coupled-stripe laser array or a broad single stripe laser diode) is required for many laser source applications which benefit from the high output power of such a laser.
Previously, several methods for achieving single narrow diffraction limited lobe emission from large area laser diodes have been proposed but all of them have deficiencies which prevent a highly successful application. These methods are discussed in the following references: Welch, D. F. et al. "Single Lobe 'Y' coupled laser diode arrays", Electron. Lett., V. 23, p. 270 (1987); Goldberg, L. et al., U.S. Pat. No. 4,686,485 (1987); Goldberg, L. et al., "Single lobe operation of a 40-element laser array in an external ring laser cavity", Appl. Phys. lett. V. 51, p. 871 (1987); Goldberg, L. et al., "Injection locking and single mode fiber coupling of a 40-Element laser diode array", Appl. Phys. Lett., V. 50, p. 1713 (1987); and Chang-Hasnein, C. J., et al. "High power with high efficiency in a narrow single lobed beam from a diode laser array in an external cavity", Appl. Phys. Lett., V. 50, p. 1465 (1987).
One method offers array designs resulting in highest laser optical gain for the lowest order spatial mode. This method is described in the above-referenced paper by Welch. In this method all array elements operate with the same phase. These designs, however, produce arrays emitting in a single lobe pattern only at relatively low power (less than 200 milliwatts in continuous wave operation) and emitting in a double lobe pattern at high power. Furthermore, these designs demand an increase in the fabrication complexity and have a greater sensitivity to fabrication and dimensional inaccuracies which occur during the fabrication process.
Another method offers laser arrays and broad stripe lasers that are injection locked using an external master laser to achieve both narrow single far field lobe emission and single longitudinal mode emission from laser arrays and from broad stripe lasers to powers of more than 500 milliwatts. See the above-referenced Goldberg et al. patent and Goldberg et al. papers. This method suffers a drawback because it requires a second laser and a high degree of temperature stability of two lasers.
Yet another method offers external cavity configurations comprising a spatial filter, a focusing lens, and a mirror which force an array to operate in a narrow far field lobe whereas without the cavity the array operates in a broad lobe. See Chang-Hasnein, C. J., et al., "High power with high efficiency in a narrow single lobed beam from a diode laser array in an external cavity", cited supra. Single lobe and multimode operation up to about 300 milliwatts have been observed. Still higher power is desired. The drawback of the latter technique is that it has not been demonstrated to be capable of achieving single lobe and high power at the same time, and does not result in a single longitudinal mode operation.
There is a manifest need to devise a high-power laser source which emits radiation in a diffraction-limited far field lobe. Such radiation can be collimated or focused into a diffraction-limited spot, as might be desired or required for coupling radiation into one or more single mode optical fibers, or collimated for free space beam propagation optical communications systems. A diffraction limited high-power laser diode source is also needed for second harmonic generation and nonlinear mixing of radiation generating optical wavelengths different than the source emission wavelengths. Furthermore, there is need for an external cavity arrangement which can be used to force a double lobe or a broad-lobe laser source to emit in a single mode while emitting in a narrow single diffraction limited far field lobe.